We have investigated the optical properties of unstrained GaAs/AlxGa1-xAs quantum dot/well systems with the aim of studying the influence of confinement on the effective exciton mass, as determined from the photoluminescence line shift in high magnetic fields (<= 50 T). The effective exciton mass is found to be more than twice the value for bulk GaAs. We attribute this to an enhanced nonparabolicity in the GaAs conduction band at the nanoscale.